A PLC (Programmable Logic Controller) is a factor essential for factory automation, and may be variably used in industrial fields. A user program for PLC may be modified or added as required, and the modified user program must be executed by being downloaded to the PLC. At this time, there is a need to modify or add functions of user program without stopping of control system. To this end, a space (allowance or clearance) is arranged at a memory block mounted on each user program, a modified portion is loaded on the space to enable the user program to be modified even during PLC operation.
However, size of memory block allocated to each user program is conventionally fixed regardless of size of each user program to a disadvantage of deteriorated utilization of non-used space, and to a disadvantage of no more enablement of program modification when there is obtained no space for modifying programs.